This invention relates to a method of forming a heterojunction of a carbon nanotube and a different material and, in particular, to a method of forming a heterojunction of a carbon nanotube and carbide.
This invention relates also to a filament, a method of inducing an electric current therein, and a method of working the same and, in particular, to a filament having a nanostructure and adapted for use in a micromachine and an electron source, a method of inducing an electric current therein, and a method of working the same.
A so-called heterojunction formed by heterogeneous or different materials is an important structure in order to utilize material-specific characteristics in an electronic device.
A carbon nanotube comprises a graphite sheet composed of six-member carbon rings and has a cylindrical structure formed by rolling the graphite sheet in a manner such that the six-member carbon rings are aligned in a helical fashion.
The carbon nanotube, together with a spherical fullerene represented by C.sub.60, is expected as a useful material for an electronic device because of its specific electric characteristics. Particularly, attention is directed to a bond of the carbon nanotube and carbide.
This is because carbide itself has very interesting electric characteristics. For example, SiC has semiconducting features. TiC has metallic features. Fe.sub.3 C acts as a ferromagnetic material. NbC attracts the attention as a superconducting material. BC.sub.x serves as an insulator. Thus, carbide has a wide variety of electric characteristics.
On the other hand, a single-wall carbon nanotube has specific electric characteristics. That is, the single-wall carbon nanotube acts as a semiconductor or a metal in dependence upon a diameter and a helical condition (an angle formed between an axial direction of the nanotube and an aligning direction of carbon atoms) (M. S. Dresselhaus et al "Science of Fullerenes and Carbon Nanotubes" (Academic Press, New York, 1996)). It is expected that various functional devices can be achieved by a combination of carbide and the carbon nanotube.
However, no conventional technique exists to form such heterojunction of carbide and the carbon nanotube. This is because the carbon nanotube has a very high Young's modulus and is therefore difficult to be mechanically processed or deformed
In order to produce a carbide nanorod using the carbon nanotube as a starting material, use has been made of a technique of contacting a multiwall carbon nanotube with volatile oxide such as SiO and B.sub.2 O.sub.2 or halide such as SiI.sub.4, TiI.sub.4, NbI.sub.4, and FeCl.sub.3 to cause high-temperature reaction (H. Dai et al "Synthesis and characterization of carbide nanorods", Nature, Vol. 375, pp. 769-772, (1995); D. Zhou et al "Production of silicon carbide whiskers from carbon nanoclusters", Chem. Phys. Lett., Vol. 222, pp. 233-238 (1994); W. Ran et al "Continuous synthesis and characterization of silicon carbide nanorods", Chem. Phys. Lett., Vol. 265, pp. 374-378 (1997)). Another technique is disclosed in EP 60388 A2 (1993) in which carbon fiber is transformed or converted into a SiC rod by the use of SiO vapor.
In the above-mentioned techniques of producing the carbide nanorod by the use of vapor-solid reaction, the carbon nanotube is exposed to reactive vapor to transform a whole of the carbon nanotube into carbide. Therefore, those techniques can not be applied to formation of the heterojunction. In other words, in order to realize the heterojunction, a part of the carbon nanotube must be selectively transformed into carbide with a remaining part protected from the reaction. However, no conventional technique can achieve such selective reaction.
Since a single-wall carbon nanotube (SWCNT) having a nanostructure has been discovered (Iijima et al, "Pentagons, heptagons and negative curvature in graphite microtubule growth", Nature, vol. 356, p776, (1992)), physical properties of the SWCNT are gradually revealed and research and development for practical applications are actively carried out.
The SWCNT comprises a hexagonal network graphite plane rolled into a cylindrical shape. The SWCNT has an electron structure widely varied depending upon a tube diameter and a chiral angle. Therefore, the electric conductivity of the SWCNT is variable between that of a metal and that of a semiconductor. The SWCNT is believed to have a feature similar to one-dimensional electric conductivity.
For example, the SWCNT is applicable to a filament having a nanostructure. For use as the filament, the SWCNT must be deformed into a desired shape. A technique of selectively deforming the SWCNT is expected to be useful in application to micromachines and in facilitating the preparation of high-resolution probes (see H. Dai et al "Nanotubes as nanoprobes in scanning probe microscopy", Nature, Vol. 384, pp. 147-150 (1996) and S. S. Wong et al "Covalently functionalized nanotubes as nanometre-sized probes in chemistry and biology", Nature, Vol. 394, pp. 52-55 (1998)).
On the other hand, a technique of selectively feeding an electric current to the filament having a nanostructure, such as the SWCNT, shows a possibility of development of electronic devices having a microstructure (S. J. Tans et al "Room-temperature transistor based on a single carbon nanotube", Nature, Vol. 393, pp. 49-52 (1998)). In addition, this technique is useful as one of the high-resolution techniques in analysis evaluation. Thus, this field of technique is highly expected
To meet such expectation, proposal is made of a filament of a field emission type (Jean-Marc Bonard et al "Field emission from single-wall carbon nanotube films", Appl. Phys. Lett. Vol. 73, pp. 918-920 (1998)). The filament comprises a plurality of SWCNT filaments scattered over a plurality of electrodes formed on a substrate. By applying a predetermined voltage between the electrodes, electrons are emitted from the filaments.
As compared with a typical thermionic emission type, the above-mentioned filament is advantageous in the following respects. Specifically, heating is unnecessary so that energy efficiency is high. The filament comprises carbon atoms alone and is manufactured at a low cost. In recent years, much attention is directed to this field of technique.
In order to individually and selectively deform the filament, for example, a manipulation technique is necessary. Manipulation of those filaments using the SWCNTs and having a nanostructure requires high resolution comparable to that required in manipulation of atoms. Therefore, it is in fact impossible to selectively deform the filament.
In addition, there is no existing technique of selectively inducing an electric current in the filament of a nanostructure. Thus, it is impossible to selectively induce the electric current in the filament using the SWCNT having a nanostructure.